Private Key
Also: secret key, privkey
cryptography · beginner
A 256-bit secret number. Whoever knows it can spend the bitcoin at the corresponding address.
A Bitcoin private key is a randomly chosen number between 1 and roughly 2²⁵⁶. From that number, elliptic-curve multiplication (on the secp256k1 curve) deterministically produces a public key, and a hash of the public key produces an address. The math runs in one direction: private → public → address. Going backward is not feasible with any known computer.
Whoever knows the private key can sign transactions spending from the address. There is no password reset, no support line, no "I lost it." Self-custody means accepting that responsibility. The flip side is that nobody else can move your bitcoin either — not a bank, not a government, not Coinbase. Holding the key is what makes the bitcoin *yours*.
In practice, you rarely write down a raw 256-bit number. Wallets use seed phrases (BIP-39) — 12 or 24 English words that encode the entropy used to derive a tree of private keys (BIP-32, BIP-44). Lose the seed phrase, lose the coins. That trade is the entire deal.